Iron

Sam using iron to dissipate a ghost.

Iron used to repel spirits In theory, if ghosts do exist and they do use electromagnetic energy to manifest, then hypothetically one could use a conductor of electricity, such as iron to drain or ground out the energy from the spirit. Therefore, touching a grounded, iron object to the spirit itself would drain all usable energy from that spirit. Based on this theory, many cemeteries are enclosed by iron fencing to keep the spirits in. Also, it has been said than an iron horseshoe placed at your front door will bring good luck by keeping evil spirits out. – Pad of Definitions (1.19 Provenance), Official Website

Sam and Dean feel cold iron is the best way to repel a spirit. They usually employ fireplace pokers, and on at least one occasion a wrench. Their hunting relatives also made use of iron. A house belonging to the Campbells, Mary's family, was outfitted with iron fixtures. John owned an iron axe that is eventually used by Sam to behead Leshii.

Sam and Dean gain firsthand knowledge of the effect of iron on spirits when they astral project with the assistance of Pamela Barnes in 4.15 Death Takes a Holiday. They find themselves trapped when a demon, despite being burned by the metal itself, encircles them with an iron chain.

Episodes

1.17 Hell House

Sam and Dean get Harry Spengler and Ed Zeddmore to post on their website, Hell Hound's Lair, that the tulpa Mordechai Murdoch can be killed by special wrought-iron rounds.

1.18 Something Wicked

Dean learned from John that a shtriga can be killed by consecrated wrought-iron rounds.

1.19 Provenance

Sam uses an iron poker to repel Melanie Merchant.

2.06 No Exit

Jo uses a knife with an iron blade, that used to belong to her father to repel the spirit of Dr. H.H. Holmes.

2.21 All Hell Breaks Loose: Part One

Sam uses an iron bar to repel the Acheri demon summoned by Ava Wilson.

2.22 All Hell Breaks Loose: Part Two

A devil's trap constructed from iron railroad lines by Samuel Colt keeps the Yellow-Eyed Demon from approaching the Devil's Gate in Wyoming until it is broken when the gate opens.

4.01 Lazarus Rising

Dean mentions iron as one of the weapons he and Bobby brought with them to summon Castiel.

4.02 Are You There, God? It's Me, Dean Winchester

Bobby's panic room, first shown in this episode, is made of iron walls coated with salt and has an iron devil's trap over the light source and fan in the ceiling.

4.06 Yellow Fever

Sam and Bobby use an iron chain etched with spell work to trap and drag the buruburu Luther Garland behind the Impala.

5.10 Abandon All Hope...

Sam, Dean and Ellen build a bomb from propane, salt, and iron nails at Jo's insistence to destroy the hellhounds surrounding them.

6.09 Clap Your Hands If You Believe...

Sam shoots the leprechaun Wayne Whittaker with iron buckshot, weakening him.

6.18 Frontierland

The phoenix Elias Finch was unable to escape from the jail or handcuffs because they were made of iron.

7.07 The Mentalists

When Melanie Golden and Camille Thibodeaux are being attacked by the ghost of Margaret Fox, Sam tells Melanie to find some iron. Melanie grabs a fire poker, but is knocked out Margaret and is unable to use it against the ghost.

7.10 Death's Door

As Bobby lays dying in a hospital bed, he travels through his subconscious, reliving various memories. In one memory he is attacked by Lara Coggins, a vengeful spirit. Before she can kill Bobby, Rufus dissipates her with iron.

7.23 Survival of the Fittest

When is possessing the motel maid Louise, Bobby attempts to pick up a mallet from the back of a pickup truck, however it is iron and causes Bobby's ghost to leave Louise's body.

9.14 Captives

When the lights in the Bunker begin to flicker, indicating a haunting. Sam rushes to the main room and sees one of the chairs spinning on its own, he grabs a iron sword from the wall and we see the apparition slowly walking behind him. Before Sam can swing around the apparition is blown away by Dean.

10.16 Paint It Black

Crowley hands over the leader of the Grand Coven Olivette to Rowena in iron shackles, telling his mother that they will prevent Olivette from using any magic on her.

10.19 The Werther Project

In order to ensure Rowena decodes the Book of the Damned, Sam locks her in iron chains in an abandoned building. Telling her once she figures out how to remove the Mark of Cain, he will let her go.

10.21 Dark Dynasty

Rowena is still chained up in iron shackles, which she claims are diminishing her capacity to translate the Book of the Damned.

10.22 The Prisoner

Rowena is still chained up in iron shackles, much to her chagrin.

10.23 Brother's Keeper

When Rowena performs the spell to remove the Mark of Cain, the shock wave is so great that it allows her to be freed of her iron shackles, and perform magic at her full power. Freezing Crowley and Castiel where they stand, she performs the impetus bestiarum spell on Castiel, and orders him to attack Crowley.

11.03 The Bad Seed

To prevent Rowena from performing any spells, Sam places her in iron shackles.

11.07 Plush

Dean informs Sheriff Donna Hanscum of the use of salt and iron against ghosts and gives Sam an iron railroad spike. Sam later uses it to exorcise the ghost from a possessed old man and Dean uses it for the same effect on Max Johnson.

11.09 O Brother Where Art Thou?

Crowley has Rowena captured once again, and placed in iron shackles. She remains in them until she, Sam and Crowley descend to Hell to communicate with Lucifer.

11.10 The Devil in the Details

Crowley and Dean place Rowena in a witchcatcher, in order to control her.

11.23 Alpha and Omega

Dean uses a fire iron while fending off the vengeful spirits in Waverly Hills Sanatorium.

12.03 The Foundry

Dean uses an iron crowbar to swat away the ghost of Lucas Kellinger after he grabs a hold of Mary's arm. Later when Mary is possessed by the vengeful spirit of Hugo Moriarty, Dean wraps her up in an iron chain to subdue Hugo's spirit.

13.05 Advanced Thanatology

Sam uses an iron crowbar to dissipate Dr. Avery Meadows when he is attacking Dean.

13.16 Scoobynatural

Sam and Dean uses iron candlestick holders to send the Phantom away after it attacks the Scooby Gang. Later, Fred's plan to trap the Phantom implements an iron chain to trap it in the room.

14.20 Moriah

When the souls of the damned begin possessing the corpses buried in the cemetery they were released in, Dean removes two rusted iron bars from a nearby fence for him and Sam to use as weapons.

15.01 Back and to the Future

Sam and Dean use rusty iron bars to beat on the damned souls possessed corpses, forcing them to flee on contact with the iron. Dean later uses an iron crowbar to dissipate the ghost of Constance Welch when she attacks him and Belphegor.

15.02 Raising Hell

Arthur Ketch uses iron pellets from a weapon he stole from the British Men of Letters against the Hell Ghosts.

The Winchesters and later Ketch wear chains made of iron around their necks to avoid being possessed by the hell ghosts. After losing his chain, Arthur is possessed by Francis Tumblety, Dean shoots him twice with iron bullets, dispossessing Arthur. Castiel is later unable to heal the wounds and Dean apologizes, stating he couldn't think of another way to save him. Arthur takes Dean's actions with good grace.

15.03 The Rupture

The Winchesters continue wearing iron chains to ward off ghost possession. Later, after being informed by Rowena that he can't protect himself with a salt circle while doing his part in the Sanetur Acre Vulnus spell, Dean arms himself with an iron crowbar instead. However, he never ends up having to use it.

Wrought Iron Lore

Wrought iron is sometimes asserted to repel, contain, or harm ghosts, fairies, witches, and/or other malevolent supernatural creatures. This belief continued into later superstitions in a number of forms:

• Nailing an iron horseshoe to a door was said to repel evil spirits or later, to bring good luck.
• Surrounding a cemetery with an iron fence was thought to contain the souls of the dead.
• Burying an iron knife under the entrance to one's home was alleged to keep witches from entering.

Cold iron is also used for similar purposes. However, cold iron is pure iron that is not smelted out of ore, but hammered pure without melting.